Recently, expectations for fuel cells as a power supply for compact electronic devices used in portable electronic devices and the like supporting information-oriented society have increased. A fuel cell is a chemical cell supplying electrons to a portable electronic device or the like by utilizing an electrochemical reaction which oxidizes fuel (such as hydrogen, methanol, ethanol, hydrazine, formalin, and formic acid) at an anode electrode and reduces oxygen in the air at a cathode electrode, and high power generation efficiency can be obtained by a single power generation device.
There are various types of such fuel cells, depending on differences in structure and fuel supplied thereto. Above all, a direct methanol fuel cell (DMFC) can generate electric power by supplying an aqueous methanol solution to an anode electrode and directly extracting protons and electrons from the aqueous methanol solution, and thus has an advantage of not requiring a reformer.
In addition, since the DMFC uses the aqueous methanol solution, which is a liquid under atmospheric pressure and has a high volume energy density, as fuel, it can handle the fuel using a compact fuel container and is also excellent in terms of safety, when compared with a fuel cell supplying hydrogen using a high-pressure gas cylinder. Thus, the DMFC has attracting attention for application to a compact power supply, in particular, for application as an alternative to a secondary cell for compact portable electronic devices.
Further, since the DMFC uses the fuel that is a liquid under atmospheric pressure, it can use a narrow curved space portion, which would be a dead space in other fuel cells, as a space for arranging a fuel container. Therefore, in an electronic device provided with the DMFC, a fuel container can be installed inside the electronic device with no restrictions on design.
Further, there is a possibility that liquid fuel which has a higher volume energy density and a higher flash point and is excellent in safety, such as ethanol and propanol, will be able to be utilized as fuel for the DMFC in the future, in addition to methanol.
Taking the DMFC as an example, an electrochemical reaction that occurs at an anode electrode and a cathode electrode within a fuel cell will be described. In the DMFC, methanol supplied through a fuel flow channel is oxidized at the anode electrode, and thereby separated into carbon dioxide, protons, and electrons as represented by the following reaction formula:Anode electrode: CH3OH+H2O→CO2+6H++6e−.
Subsequently, a flow of electrons generated when the electrons produced at the anode electrode move to the cathode electrode through an external load is extracted as electric power. On the other hand, the protons produced at the anode electrode are transported to the cathode electrode side through an electrolyte membrane. At the cathode electrode, the protons permeating through the electrolyte membrane react with an oxidizing agent as represented by a reaction formula described below, to produce water. As an oxidizing agent used for the DMFC, air that is abundant in an external atmosphere is often used.Cathode electrode: O2+4H++4e−→2H2O.
Except for a high-temperature fuel cell such as a molten carbonate cell, fuel cells such as a solid polymer fuel cell, a solid oxide fuel cell, a direct methanol fuel cell, and an alkaline fuel cell have a plane-stacked structure obtained by stacking an anode separator in which an anode flow channel for supplying a reducing agent is formed, an anode current collector collecting electrons from an anode catalyst layer, an anode gas diffusion layer, the anode catalyst layer, an electrolyte membrane, a cathode catalyst layer, a cathode gas diffusion layer, a cathode current collector feeding electrons to the cathode catalyst layer, and a cathode separator in which a cathode flow channel for supplying an oxidizing agent is formed, in this order.
In particular, among the components of the fuel cell as described above, a composite including the anode catalyst layer, the electrolyte membrane, and the cathode catalyst layer formed by means such as thermocompression bonding or the like is called a membrane electrode assembly (MEA), which is a minimum constituent unit in constituting the fuel cell.
Further, a material having electrical conductivity is used for the anode separator and the cathode separator, because the anode separator may serve as an anode current collector and the cathode separator may serve as a cathode current collector. Furthermore, the anode separator may play a role of supplying the reducing agent to the anode catalyst layer, and the cathode separator may play a role of supplying the oxidizing agent to the cathode catalyst layer.
Any of the various types of fuel cells described above can produce a high current. However, since these fuel cells have a low output density per volume, it is necessary to improve output density to utilize them as a compact power supply.
Accordingly, generally, a plurality of fuel cells are stacked such that anode electrodes and cathode electrodes thereof are alternately brought into contact (hereinafter, such a stacked structure will also be referred to as a “fuel cell stack”), connected in series to increase an output voltage thereof, and thereafter mounted in an electronic device.
However, if contact resistance between the fuel cells is increased in the above fuel cell stack, internal resistance is increased, causing a reduction in overall power generation efficiency. In a conventional fuel cell stack, contact resistance between fuel cells in the fuel cell stack is suppressed and electrical conductivity is improved by providing a sealing material for sealing a reducing agent and a sealing material for sealing an oxidizing agent for each separator to improve sealing performance, and by closely maintaining electrical contact between an anode gas diffusion layer and an anode separator serving as an anode current collector and between a cathode gas diffusion layer and a cathode separator serving as a cathode current collector. In addition, both ends of the fuel cell stack are pressed down using fastening members such as a thick and rigid presser, a bolt, and a nut. However, since the fuel cell stack is provided with these fastening members, the size and weight of the fuel cell stack are increased, which has caused a problem that output density of the fuel cell stack is reduced.
Further, the conventional fuel cell stack has another problem that its output density is reduced because the anode separator and the cathode separator have too large thicknesses. It is necessary to form an anode flow channel for uniformly supplying a reducing agent to an entire surface of an anode catalyst layer, in the anode separator, and it is also necessary to form a cathode flow channel for uniformly supplying an oxidizing agent to an entire surface of a cathode catalyst layer, in the cathode separator.
If the thicknesses of the anode separator and the cathode separator are reduced by narrowing thicknesses of the anode flow channel and the cathode flow channel, pressure losses thereof when supplying the reducing agent and the oxidizing agent are increased. Therefore, the size of auxiliary equipment such as a pump and a fan for supplying the reducing agent and the oxidizing agent has to be increased, and as a result, the output density of the fuel cell stack is reduced. In addition, power consumption by the auxiliary equipment provided to the fuel cell stack is also increased, causing a reduction in power generation efficiency.